Ground effect flying craft, wind powered

[Jeremy Harris]

The HP aircraft took a lot of development and even then only really became feasible when technology gave them the materials needed (like Tedlar for the wing skins, etched alloy tapered tubes for the spars, expanded polystyrene for the wing ribs). For centuries people had dreamt of human powered flight, or even tried to build machines, but it wasn't until we developed materials, and a better understanding of aerodynamics, that these things became a reality.

HP aircraft still took years of development to get to the point where we can build one pretty easily, with pretty much off-the-shelf materials. If anything, they've got bigger, because the advent of things like stiff and light carbon fibre spars has allowed a bigger span than the tapered, etched alloy spars that the Gossamer series used at first.

Same goes for prop design, although the Wrights did pretty well - there props on the Flyer were around 80%, which is better than those on most aircraft flying today. HP aircraft have pushed the prop efficiency boundary up to around 90%, which is probably pretty close to the theoretical maximum (you can never get 100%, because a prop blade will always have some drag).

[Tiny Turnip]

It is worth noting, from a practical point of view, that MacCready's HP aircraft, as Jeremy says, didn't pursue ground effect exhaustively. Both Gossamer Condor and Gossamer Albatross were high wing designs. With Gossamer Albatross, the cross channel HPA, There is a recognition that drag caused by turbulence close to the water/wave surface was significant, and that Bryan Allen, the pilot, found the pedaling easier with a little more altitude.

Morton Grosser's detailed book, Gossamer Odyssey, is on Google books.

[Tiny Turnip]

I should note that the turbulence issue is associated in particular with the very low power levels available, and is likely to be a much less significant issue with wind power.

[Tiny Turnip]

With both aircraft, incidentally, MacCready was achieving wing spans of nearly 30m with unladen weights of only 31, 32 kgs, more than 30 years ago now.

[Tiny Turnip]

There's an interesting video here:

http://www.youtube.com/watch?v=F4zEaYl01Uw

of a NASA concept for a dynamic soaring platform using wind gradients on the ocean surface (above waves / between troughs) modelled on albatross flight.

Incidentally, dynamic soaring by sailplane has now been measured at an incredible 468mph.

[A.T.]

Originally Posted by Jeremy Harris

When you see sea birds doing this they are not getting energy from the wind in a steady state, they are doing something called dynamic soaring. They are small enough to exploit the wave-induced updrafts and downdrafts to gain energy

Dynamic soaring doesn't require any vertical flows (updrafts and downdrafts). It works with purely horizontal flows, if there is a velocity gradient (which is always the case to some extend).



Here a very detailed article:
http://www.howfliesthealbatross.com/...%20Soaring.pdf

Using updrafts and downdrafts is usually not called "dynamic soaring" but "static soaring":
http://en.wikipedia.org/wiki/Dynamic_soaring

1) that the course is not horizontal,
2) that the wind is not horizontal, or
3) that the wind is not uniform.
(...)
The first case-described above by Rayleigh is simple gliding flight, the second is static soaring (using thermals, lee waves or slope soaring), and the last is dynamic soaring.[1]

Originally Posted by Jeremy Harris

effectively they are extracting wave energy from the wind.

The waves are just obstacles which increase the gradient. But even over a rather flat surface you have a gradient, due to friction.

Originally Posted by Jeremy Harris

What I wrote before is correct, if you want to extract energy from the wind then you have to create a differential velocity between the vehicle/aircraft/vessel and the wind. If the vehicle is free flying in a steady state then it will always be in the moving body of air and unable to extract energy from the wind.

This is correct for uniform wind. But as I said there is always a gradient:
http://en.wikipedia.org/wiki/Wind_gradient

A free flying vehicle (on constant course and altitude) could (at least theoretically) exploit the velocity difference between the airmass layers, just like a sailboat exploits the velocity difference between air and water. You could use a sail on a ground effect glider (acting as the keel in the slow air) and then a kite in the fast air (acting as the sail). Or use two kites at different altitudes.

But it would have to be super efficient to actually work.

[P Flados]

The wind gradient from surface up would be tough to exploit.

With wind over water, it only takes a small surface below the water (dagger board) to allow you to create forward thrust from a large sail area. This is with a pretty good velocity difference between adjacent mediums.

The wind gradient you seem to be wanting to exploit is a thin region of air close to the surface being one region and the air a good ways up would be the other region.

The most efficient way to extract energy from air is a sail or fixed wing as compared to a propeller.

The surface area needed to get any real forward thrust would need to be pretty large vertical surfaces for both an upper and a lower. It would also only offer a decent speed differential when the true wind speed is quite high.

To get a large surface area in the thin layer close to the surface, the aspect ratio would be very not good. The combined weight of the two vertical wing/sail surfaces would be much higher than the sail / dagger board combination and the forward thrust would be lower.

If you want to "fly" using wind your choices are limited. Here are a few items that come to mind in the context of flying sailboats.

• Get something like a Moth or Rave. Some make claims of flying for this type of craft. Although what I would call "foiling" works similar to "flying", getting the lift from a foil traveling through water is not the same as having a craft supporting itself with lift generated by air flowing over wings

• Make a boat that goes very fast and use a crossbeam or other near horizontal surface(s) to generate the lift that keeps the craft up. I will bet that the wing assembly on VSR2 provides more upward force than the total weight of the boat. However, the forward pod is designed as a planing surface that really does need to stay in near constant contact with the water. VSR2 shows that this approach is possible, but no one has shown that it is practical.

• Go with a craft that uses hydrogen or helium for lift. This idea has been around for a while (look up Bernard Smith) but I do not think it has really been tried. This would have some real potential for scaling up the successes of the kite boarders.

• A low speed large diameter under the water prop angled up coupled with a wind turbine could potentially generate enough thrust to support a craft, but only at the expense of using up power that would be better spent making forward progress. Turbine driven boats really do work, but they do not have excess power to spend on dynamic generation of lift. Simple displacement hulls work fine

.

[Jeremy Harris]

Originally Posted by A.T.

Dynamic soaring doesn't require any vertical flows (updrafts and downdrafts). It works with purely horizontal flows, if there is a velocity gradient (which is always the case to some extend).......rest snipped for brevity.......

I agree. Oversimplification on a forum post is always an invitation to someone to post a lengthy "correction", which is why I referred the OP to a web search on dynamic soaring for a better understanding of the principle..........

I was trying to keep the answer brief, but did mention the vertical movement and velocity change needed.